TOLERANCE OF DEPRIVATION OF AIR.

265

system. But a very slight irritation is sufficient to produce respiratory movements; the heart's action is quickened; and the animal for a time shows an increase of its general activity.

310. Animals will in general bear deprivation of air well or badly, according as the respiration is more or less active. Thus a warmm-blooded animal usually dies, if kept beneath water for more than a few minutes; though there are some which are enabled, by peculiar means, to sustain life much longer (§ 265). In cold-blooded animals, however, whose demand for oxygen is much less energetic, this treatment may be continued for a much longer time without the loss of life. Thus the common Water-Newt naturally passes a quarter of an hour or more beneath the surface, without coming up to breathe; and it may be kept down for many times that period without serious injury. And, as we might expect from their peculiar condition, warm-blooded animals, when hybernating, may be kept beneath water for an hour or more, without apparent suffering; although the same animals, in their active state, would not survive above three minutes. There is reason to believe that a similar condition may be produced in Man, by the influence of mental emotion, or of a blow on the head, at the moment of falling into the water; so that recovery is by no means hopeless, even though the individual may have been more than half an hour beneath the

surface.

Structure and Actions of the Respiratory Apparatus.

311. In animals whose organization is most simple, the act of respiration is not performed by any organ expressly set apart for it; but it is effected by all the parts of the body that are in contact with the element in which the animal lives, and from which they derive their necessary supply of oxygen. This is the case, for example, in the lower class of Animalcules, in the Polypes, Jelly-fish, Entozoa, and many other animals. Even in the higher classes, a considerable amount of respiratory action takes place through the skin, especially when it is soft and but little covered with hair, scales, &c., as in Man, and in the Frog tribe; but we almost invariably find in them a prolongation of this membrane, specially designed to enable the blood and the air to act upon each other, and having

266

STRUCTURE OF RESPIRATORY ORGANS.

its structure modified for the advantageous performance of this function. This modification consists in the peculiar vascularity of this membrane, that is, in the large number of vessels that traverse its surface; and also in the thinness of the membrane itself, by which gases are enabled to permeate it the more readily. Moreover, we always find this membrane so arranged, that it exposes a very large surface to the air or water which comes into contact with it; and this surface may be immensely extended, without any increase in the size of the organ. Thus the small lungs of a Rabbit really expose a much larger respiratory surface to the air, than is afforded by the large air-sacs of a Turtle which are ten times their size. This is effected by the minuteness of the subdivision of the former into small cavities or air-cells, whilst the latter remain as almost undivided bags.

312. It is desirable to possess a distinct idea of the mode in which the surface is thus extended by subdivision. We may, for the purpose of explanation, compare the lung to a chamber, on the walls of which the blood is distributed, and to the interior of which the air is admitted. This chamber, for the sake of convenience of description, we shall suppose to

Now if a parti

have two long and two short sides, as at a. tion be built-up in the direction of its length, as at B, a new surface will be added, equal to that which the two sides previously exposed; since both the surfaces of this partition are supplied with blood, and are exposed to the air. Again, if another partition be built-up across the chamber, as at c, a new surface will be added, equal to that which the ends of the chamber previously exposed. And thus, by the subdivision of the first chamber into four smaller ones, the extent of surface has been doubled. Now if each of these small ones were divided in the same manner, the surface would again be doubled; and thus, by a continual process of subdivision, the

STRUCTURE OF RESPIRATORY ORGANS.

267

surface may be increased to almost any extent compatible with the free access of air to the cavities, and of blood to the walls. In the same manner, where the respiratory membrane is prolonged outwardly, so as to form gills, which hang from the exterior of the body (as is the case in most aquatic animals), its surface is very much extended by disposing it in folds, and by dividing these folds into fringes of separate filaments. It has been calculated that, by this kind of arrangement, the gills of the Skate present a surface four times as great as that of the Human body.

313. The structure and arrangement of the Respiratory organs differ, according as they are destined to come in contact with air in the state of gas, or to act upon water in which a certain amount of air is dissolved. In the former case, we usually find the respiratory membrane (which is but a prolongation of the skin or general envelope) forming the wall of an internal cavity,—just in the same manner as the membrane, through which the act of absorption takes place in animals, is prolonged from the skin so as to form the wall of the digestive cavity (§ 14). Such a cavity for the reception of air into the interior of the body, exists in all air-breathing animals; and in the Vertebrata it receives the name of lung. On the other hand, in animals that breathe by means of water, the respiratory surface is prolonged externally, so as to be evidently but an extension of the general surface, just in the same manner as the roots of plants are prolonged into the soil around them. These prolongations, termed branchiæ or gills, which may have various forms, carry the blood to meet the surrounding water, and to be acted-on by the air it contains; and then return it to the body in a purified condition.

314. The form and arrangement of the gills vary greatly in the different classes of aquatic animals. Sometimes they simply consist of little leaf-like appendages, which have a texture rather more delicate than that of the rest of the skin, and which receive a larger quantity of blood. In other instances, they are composed of a number of branching tufts, which are more copiously supplied with vessels. Among the ANNELIDA we observe a great variety in the mode in which these tufts are disposed; and this is connected with the general habits of the animal. Thus in the Serpula (fig. 145), whose body is inclosed in a tube, the tufts are disposed

268

RESPIRATORY ORGANS OF AQUATIC ANIMALS.

around the head alone, and spread out widely into the sem

blance of a flower.

In the Nereis (fig. 52) and its allies, they

are set upon nearly every division of the body, and are much smaller. Their usual arrangement in these marine worms may be seen in fig. 146, which represents one of the appendages of Eunice. The tuft of gills is shown at b; at c is seen a bristleshaped filament, which may perhaps be regarded as the rudiment of a leg; and the projections to which the letters t and ci point, also seem connected with the movements of the animal. In the Arenicola (the lobworm of fishermen) we find the respiratory tufts disposed on certain segments only, and possessing more of an arborescent (treelike) form (fig. 147).

Fig. 145.

b

t

ci

315. A somewhat similar arrangement is seen in the larvæ of many aquatic INSECTS, which breathe by means of gills; although all perfect Insects breathe air in the manner to be presently described. In fig. 148 is represented the larva of the Ephemera (Day-fly), which breathes by means of a series of gill-tufts disposed along the abdomen, and also prolonged as a tail. In the CRUSTACEA, we usually find the gills presenting the form of flattened leaves or plates. In the lower tribes of the class, they project from the surface of the body; but in the higher, they are inclosed within a cavity, through which a stream of water is made con

Fig. 146.

GILL-TUFT OF EUNICE.

Fig. 147.

ARENICOLA.

RESPIRATORY ORGANS OF AQUATIC ANIMALS.

269

stantly to flow, by mechanism adapted for the purpose. Their form and position in the Crab are shown at b, b', fig. 47. Although these animals usually reside in the water, or only quit it occasionally, there are some species, known under the name of land-crabs, which have the power of living for some time at a distance from water. In order to prevent their gills from drying up, which would destroy their power of acting on the air, there is a kind of spongy structure in the gill-chamber, by which a fluid is secreted that keeps them constantly moist.

Fig. 148. LARVA OF EPHEMERA.

316. In the MOLLUSCA We find the gills arranged in a great variety of modes. In the lowest class, the TUNICATA, the respiratory membrane is merely the lining of the large chamber formed by the mantle (fig. 63), through which a stream of water is continually made to flow by ciliary action (§ 319); and this surface is sometimes extended by the folding or plaiting of the membrane. In most of the CONCHIFERA, however, we find four lamella or folds of membrane

Fig. 149.-RESPIRATORY APPARATUS OF THE OYSTER.

1, one of the valves of the shell; , its hinge; m, one of the lobes of the mantle; m', a portion of the other lobe folded back; c, muscles of the shell; br, gills; b, mouth; t, tentacula, or prolonged lips; f, liver; i, intestine; a, anus co heart.